CDs and DVDs are prevalent as optical disks. The development of a next-generation optical disk using blue laser has been advanced. A further increase in capacity has been required continuously. Super multidrive having the function of performing not only playback of a CD by one optical disk apparatus but also playback of a CD and a DVD and performing recording for CD-R/RW, DVD-RAM, and DVD-R/RW disks has been commercialized. This has been considered to become a predominant product.
Since a PRML (Partial Response Maximum Likelihood) method is excellent in improvement effect of an S/N ratio, it has been in widespread use as a means for bringing a magnetic disk into a great increase in capacity. The PRML method is a decoding technique which compares N consecutive signal samples with the target signal and decode to a most likelihood bit stream. Viterbi decoding method, which is one ML method, has been brought into wide practical use, because it can greatly reduce the circuit scale. Although a direct slice method has been used as a method for reproducing or playing back an optical disk from long ago, there appears to be a limit to speeding up and an increase in capacity. Therefore, the PRML method is being also applied for optical disk playback means.
With the PRML method intended for the optical disk as one example, a technique for allowing a target signal level of a Viterbi decoder to adaptively follow the level of a readout signal so as to match the level of the readout signal has been disclosed in JP-A No. 296987/1999 (Patent Document 1). FIG. 2 shows the construction of the Viterbi decoder capable of setting target levels. The Viterbi decoder 40 comprises a BR unit 41, an ACS (Add Compare Select) unit 42, a path memory 43, a target level table 44, and a target study unit 45. An EQ wave 53 subjected to equalization processing is inputted to the BR unit 41, where a branch metric value relative to a target level is calculated for each bit string. At this time, the target level is instructed from the target level table 44. The ACS unit 42 adds the branch metric values corresponding to the respective bit strings to state metric values (ones obtained by sequentially adding the branch metric values with the transition of states and processed so as not to cause their diversion) at a state at the instant preceding one time and respective states. Here, one is selected from the transition processes lasting to the present time whose state metric is smaller. (the number of the transitions processes is normally two or might be one depending upon run-length restriction). The term state means a bit string stored with respect to the transition of one time. When a PR class with a constraint length is 4 is taken, for example, the bit string is expressed in 4 bits and the state is expressed in 3 bits. Binalized results or outputs have been stored in the path memory 43 for a sufficient long time. Upon selecting the transition processes with smaller state metric, the ACS unit 42 rearranges the information stored in the path memory according to the result of selection of each state metric value. With repetition of such processing, the information stored in the path memory are gradually integrated so that so-called path merge is completed in which they reach the same value without depending upon the bit strings after the passage of a sufficient long time. A binary output 51 is binary information taken out from the termination of the path memory for each time. The target study unit 45 has the function of extracting a bit string from the binary output 51 whose length is equal to the constraint length, averaging the EQ wave 53 by every bit strings and updating each target level stored in the target level table 44. The target level table 44 also includes the function of setting target level values for each bit strings in advance in response to a pre-set value load command 56 issued from a CPU.
An AD converter mentioned above performs conversion to a digital signal string in sync with a clock signal generated from a readout signal by using a PLL (Phase Locked Loop) circuit. Thus, the generation of a stable clock from a PLL is a prerequisite for carrying out stable binarization in the PRML method.
As one example of a next-generation optical disk using blue laser, one having a memory capacity of 23.3 GB/25 GB as a Blue-ray Disc recorder has been commercialized. Its further increase in capacity in future is expected. An optical disk apparatus compatible with a Blu-ray Disc and an optical disk apparatus with increased capacity may face problems such as degradation of the precision of a clock generated by a PLL and an inability to make a proper edge decision where the amplitude of the shortest run-length signal is significantly reduced.
As a technique for solving instability of the PLL due to the reduction in the amplitude of the shortest run-length signal, there has been known a method for inserting means for pre-decoding stage prior to the PLL as disclosed in each of JP-A No. 175673/2002 (corresponding U.S. Pat. No. 6,788,484/Patent Document 2) and JP-A 172250/1998 (Patent Document 3). As disclosed in JP-A No. 182335/2000 (Patent Document 4), there has also been known a method for performing phase detection at other target level other than a zero level.